The present invention relates to a planar array antenna which can be applied to a transmit/receive antenna used for a WLL (wireless local loop) terminal.
FIGS. 5A to 5C illustrate one example of a prior art planar array antenna of the above type. Referring to these figures, a plurality of (two in this example) patch antenna elements 101 and 102 are arrayed on a rectangular dielectric substrate 100. The elements 101 and 102 are coupled to each other by a feed line 103, while the element 102 is coupled to a feeding point 105 by a feed line 104. The feed lines 103 and 104 are each constituted of a strip line adhered onto the dielectric substrate 100.
In the prior art planar array antenna, an electric power is applied, as a series feed, from the feeding point 105 to the patch antenna elements 101 and 102 through the feed lines 103 and 104.
The planar array antenna so constituted is miniaturized as a whole by the dielectric effect of the dielectric substrate 100. Since, however, the antenna is decreased in gain due to a dielectric loss, a usable bandwidth of VSWR (voltage standing-wave ratio) is narrowed. Since, moreover, the plurality of patch antenna elements 101 and 102 are arrayed and an electric power is applied to these elements as a series feed, the following problem arises. The patch antenna elements 101 and 102 are difficult to arrange at the optimum interval under the influence of a so-called contraction rate due to the dielectric of the dielectric substrate 100. This problem will be described more specifically.
As illustrated in FIGS. 5A and 5B, the electrical length of the antenna is determined such that the length of each of the patch antenna elements 101 and 102 and the interval between them are both .lambda./2 when the wavelength of transmitted/received wave is .lambda.. In FIGS. 5A and 5B, it is .lambda./2 and P=.lambda. that correspond to the electrical length. The contraction rate, which is one of dielectric effects of the dielectric substrate 100, is taken into consideration in order to set the electrical length.
Assuming that Teflon (known under the trade name of du Pont) is employed as the dielectric substrate 100 and its effective permittivity is .epsilon.e, an actual physical distance R between the patch antenna elements 101 and 102 is given by the following equation: EQU R=.lambda./2(.epsilon.e).sup.1/2.apprxeq.0.7.lambda./2
If, as shown in FIG. 5C, the energy area of the patch antenna element 101 is S101 and that of the patch antenna element 102 is S102, these areas overlap each other to cause a region S103 shaded diagonally therein. The overlapped region S103 reduces the antenna efficiency and accordingly the maximum gain cannot be obtained under the influence of a dielectric loss. When Teflon is used as the dielectric substrate 100, the gain falls within a range from 8 dBi to 9 dBi, which is about 30% lower than the maximum gain in the ideal status or in air.
If an electric power is applied to the patch antenna elements 101 and 102 as a parallel feed, the foregoing problem does not arise, whereas the following drawback occurs: since the antenna necessitates an allotter, its structure is complicated and increased in size, and a loss is produced from the allotter.